This invention relates to a brake force regulator for a motorcycle hydraulic brake system, comprising a control piston, which is slidable in response to the pressure of a hand master cylinder and adapted to reduce the brake pressure of a rear-wheel brake by arranging for the control piston to be moved into a fluid inlet chamber connected with the pedal master cylinder, such movement being caused by the pressure of the hand master cylinder under increase in volume of a fluid outlet chamber connected with the rear-wheel brake, the control piston having a fluid passageway connecting the fluid inlet chamber with the fluid outlet chamber with a closure member arranged therein, the control piston being urged into abutment with a stop on the side close to the fluid inlet chamber by a return spring and having an effective area on the side close to the fluid inlet chamber, which area is subjected to pressure against the force of the return spring to cause displacement of the control piston in the direction of the fluid outlet chamber, with the closure member having an associated piston which is movable in the closing direction of the closure member against the force of a spring extending into a hollow space connected to atmosphere.
From German Patent DE-OS No. 2,558,825 a brake system is known which is provided with a first brake-applying device connected with a front-wheel brake and a second brake-applying device connected with a rear-wheel brake. In this arrangement, the first brake-applying device is adapted to reduce a brake force deliverable to the rear-wheel brake through the second brake-applying device.
This known brake system comprises a brake force regulator which resembles conventional brake force regulators for passenger cars. Actuation of the pedal master cylinder initially results in unreduced pressure supply to a front-wheel brake and reduced pressure supply to the rear-wheel brake. If in addition the hand master cylinder is actuated, the pressure thereby generated counteracts the control force of the brake force regulator so that it reduces the pressure of the rear-wheel brake to a greater extent than if only the pedal master cylinder is actuated. In this arrangement, the brake force regulator is so designed that the displacement travel of its stepped piston is sufficient to reduce the pressure in the pressure of a front-wheel brake.
This prior known brake force regulator has the functional disadvantage that its stepped piston can be displaced also when pressure is built up by the hand master cylinder, i.e., when only the hand brake, which acts on the front wheel, is applied. This displacement of the stepped piston causes pressurized fluid to be drawn from the rear-wheel brake. As a result, a vacuum will be produced in the rear-wheel brake which may draw in air and containments that might result in a failure of the brakes. At all events, the drawing in of pressurized fluid from the rear-wheel brake will increase the brake clearance. This disadvantage is eliminated by the subject matter of the copending U.S. application of K. Engert, Ser. No. 027,980, filed Apr. 9, 1979, having the same assignee as the present application.